Memory is busy, although there is nothing on the phone?

Question

Android

On the phone, all media is on the sd card, there are almost no applications, the cache and all sorts of thumbnails have been deleted.
I looked at the size of each folder in memory, everything weighs almost nothing, I also looked at hiding the folder
. What is the memory doing?

The phone says something like this:

Answer 1

Check messengers (cache)
Telegram: Settings -> Data and memory -> Memory usage -> Clear cache Telegram
Viber: ...
...

Answer 2

if you press clear, then there will be an opportunity to see the list of applications sorted by the space they occupy (this is private memory - the cache and the actual data, they often cannot be viewed without root, but can be deleted).
in fact, these are two places where memory goes - external memory is the part that is available, for example, from the total commander or when connecting a smartphone with a wire to a computer and the private memory of the application,
I remember I found several gigabytes in the camera application, when you start the smartphone for the first time, the camera by default adds media to internal memory of the application, later all this is changed to external memory and photos and videos are forgotten to be deleted